Add case name to multimedia export

Adding case names to the multimedia export attachments. This was originally spec'ed in the export spec, but it was mistakenly changed later. There was talk of printing the entire hierarchy of case parents/grandparents, but I couldn't find a great way to do that without querying couch n+1 times.

Probably easier to be reviewed commit by commit
Might want to make this unicode (add `u` prefix) since `question_id` and `extension` are unicode.